Theoretical Study and Optimization of Apodized Fiber Bragg Grating for Single and Quasi-distributed Structural Health Monitoring Applications

In this paper, we present an apodized Fiber Bragg grating (FBG) for the single and quasi-distributed sensing applications. Optical characteristics, such as reflectivity, Full-Width at Half Maximum (FWHM), and side-lobes of FBG critical for an efficient quasi-distributed sensing networks are optimized for the proposed grating. The coupled-mode theory and transfer matrix method are utilized to establish numerical modeling of apodized FBGs for single and quasi-distributed sensing networks. All the simulations are performed using MATLAB. Simulation results illustrate that for the optimized grating parameters L = 10 mm and $\overline{\delta{n}}=0.8 \times 10^{-4}$, the proposed grating is characterized with reflectivity of 0.532, FWHM of 0.132 nm, Maximum Side-Lobe (MSL) of -36.25 dB, and Side-Lobe Suppression Ratio (SLSR) of -33.51 dB. Comparative performance analysis of the proposed grating with the elite apodization profiles, Gaussian and Tanh4z, is carried out through simulation. These results illustrate that the proposed grating has better reflectivity and FWHM as compared to Gaussian. It has better side-lobes suppression than Tanh4z apodized FBG structure as well. Generally, an FBG sensor characterized by the high reflectivity, narrower FWHM, and better side-lobes suppression is of great importance in Wavelength Division Multiplexing (WDM) quasi-distributed sensing networks. The optimized grating is utilized for five-stage WDM quasi-distributed strain and temperature sensing networks. A high dynamic strain/temperature range of 1450 µ∊/131.6°C is obtained using this optimized grating. This dynamic range of strain is very suitable for real-field structural health monitoring applications making our proposed grating a suitable candidate for the above mentioned application.